1. Field of the Invention
This invention relates generally to power supplies, and more particularly to a switching power supply which is particularly useful in circuit breaker applications.
2. Description of the Related Art
In electric power distribution systems, circuit breakers are used to protect power lines and other distribution equipment from excessive current flows which tend to overheat and damage current conductors. Circuit breakers carry and monitor the current flowing through the power distribution lines. When the current reaches an undesirable high level in a particular line, a tripping device or trip unit associated with the circuit breaker interrupts current flow in the power line, usually by energizing an electromagnetic relay connected in series with the line. Each circuit breaker includes current monitoring circuitry, trip determination circuitry, and circuitry to power the tripping device. Moreover, many circuit breakers used in industrial applications offer sophisticated delay circuitry to maximize power dissipation and to prevent unnecessary tripping, alarm or annunciation circuitry, and other types of fault detection circuitry, such as circuitry for ground fault detection.
A power supply is needed to provide adequate power to the circuitry of the circuit breaker during operation. Since circuit breakers monitor the power in the distribution lines, power is conveniently drawn from the distribution lines to provide power for the power supply. Circuit breakers of this type are commonly referred to as "fault-powered" circuit breakers. Advantageously, fault-powered circuit breakers operate over a wide range of line current to ensure proper operation of the circuit breaker. For instance, under ground fault conditions, when current is drawn from the line through an external conductor, such as when the conductor comes into contact with a motor casing, the line current is low; the power supply must still be able to adequately power its associated circuitry and the tripping relay in order to isolate the affected power line and prevent the ground fault from causing damage, antithetically, a "short" circuit may produce currents in the power line which are many times greater than the rating of the line. To prevent damage caused by this type of severe overcurrent condition, the power supply contains surge protection so that it continues providing power to the associated circuitry and to the tripping relay in order to isolate the affected power line and prevent damage to the power line and any related electrical equipment. An overcurrent condition may produce currents in excess of twenty times the current encountered during a ground fault situation, and the power supply must be able to deliver an output suitable to power the circuitry of the circuit breaker over this entire operating range.
While fault-powered circuit breakers offer many advantages over externally powered circuit breakers, they also present some drawbacks. The most simple and straightforward power supply is a linear power supply which, if providing sufficient power at minimum input levels, must waste the power at higher input levels. This inherent inefficiency of linear power supplies is overcome by switching power supplies. However, switching power supplies can distort the power signal and upset the metering function as a result of the change in load presented to the current transformer as the power supply switches. Since the power signal being monitored by the current transformer is used by the trip-determination circuitry to control the tripping device, a distorted signal reduces the accuracy of the trip-determination circuitry. The reduced accuracy can cause the circuit breaker to trip unnecessarily, or to continue to pass current during an overload condition.
Moreover, some fault-powered circuit breakers cannot provide adequate power particularly during low current situations. At such times the available power is insufficient to energize electromagnetic tripping relays which draw a significant amount of power during actuation. To overcome this disadvantage, U.S. Pat. No. 4,258,403 issued Mar. 24, 1981 to Shimp discloses a ground fault circuit interrupter which charges a timing capacitor via a summing current transformer. When the charge on the timing capacitor reaches a predetermined level, the capacitor is completely discharged through the trip coil of the circuit interrupter. The electrical current is sufficient to actuate the trip coil to open circuit the power line. Therefore, during low current situations, the circuit breaker generates enough power to open circuit the power line to prevent possible damage. However, Shimp's circuit uses the timing capacitor to power only the trip coil, so that the circuit does not supplement the power supply during a ground fault situation. Instead, the separate timing capacitor is utilized as part of the tripping circuitry, so that the tripping circuitry not only initiates the tripping signal, but also provides the power needed to actuate the trip coil.
The present invention is directed to overcome one or more of the problems set forth above.